98 



MICROSCOPIC METHODS. 



than that of methylene-blue and it approaches the violets in tint. 

 Bismarck-brown is a weak stain, but is useful for some purposes. 

 Formerly it was much used in photomicrographic work, as it 

 was less actinic than the other stains. It is not, however, 

 needed now, on account of the improved sensitiveness of plates. 

 It is most convenient to keep saturated alcoholic solutions of 

 the stains made up, and for use to filter a little into about ten 

 times its bulk of distilled water in a watch-glass. A solution, of 

 good body is thus obtained. Most bacteria (except those of 

 tubercle, leprosy, and a few others) will stain in a short time in 

 such a fluid. Watery solutions may also be made up, e.g. a 

 saturated watery solution of methylene-blue or a i per cent 

 solution of gentian-violet. Stains must always be filtered before 

 use ; otherwise there may be deposited on the preparation 

 granules which it is impossible to wash off. The violet stains 

 in solution in water have a great tendency to decompose. Only 

 small quantities should therefore be prepared at a time. 



The Staining of Cover-glass Films. — Films are made from 

 cultures as deseribed above. The cover-glass may be floated on 

 the surface of the stain in a watch-glass, 

 or the cover-glass, held in forceps with the 

 film side uppermost, may have as much 

 stain poured on it as it will hold. When 

 the preparation has been exposed for the 

 requisite time, usually a few minutes, it is 

 well washed in tap water in a bowl, or with 

 distilled water with such a simple contriv- 

 ance as that figured (Fig. 52). The figure 

 explains itself. When the film has been 

 washed, the surplus of water is drawn off 

 with a piece of filter paper, the preparation 

 is carefully dried high over a flame, a drop 

 of xylol-balsam is applied, and the cover- 

 glass mounted on a slide. It i^ sometimes 

 advantageous to examine films in a drop of 

 water in place of balsam. The films can 

 be subsequently dried and mounted perma- 

 nently. In the case of tubercle, special 

 stains are necessary (p. 104), but with this exception, practically 

 all bacterial films made from cultures can be stained in this way. 



Fig. 52. — Syphon wash- 

 bottle for distilled water used 

 in washing preparations. 



